This invention relates to the production of hollow cylinders or tubes, and, in particular, to producing a textured hollow tube by means of centrifugal slip casting anisometric materials to a preferred orientation against a liquid absorbent mold surface.
Powder processing of components such as tubes is advantageous in many circumstances, including where the melting temperature of the desired production material is sufficiently high that liquid casting and solidification of the entire component is impractical due to restrictions of the mold material or solidification direction or in cases in which phase changes or phase separation occurs on freezing the melt. Powder processing is also advantageous in other microstructural aspects such as grain size control and induced texture.
Current technology for some types of tube production relies on powder particles to be consolidated by means of cold-pressing or hot-pressing, or by traditional slip casting. Most common is cold-pressing or hot-pressing the precursor powder into a cylinder. The center of the cylinder is then removed by a machining process, such as drilling, boring, or grinding. Because many ceramic materials are difficult to machine, the removal of the center of the pressed cylinder can be very difficult and expensive. While it is possible to press a tube without having to remove the center, there is limited preferred orientation induced in this process when the tube wall thickness is small compared to the length of the tube due to die wall friction effects. Cold-pressing of materials relies completely on the rotation and fracturing of powder particles to obtain preferred orientation. Fracturing of the powder can often be detrimental because of the reduction in particle size. The amount of orientation induced by pressing varies with position inside the tube as well, also due to die wall friction effects. With axial pressing, the tube ends would obtain significant orientation with the long dimension of the particles parallel to the pressing surface. At the tube longitudinal center, minimal orientation benefits would be obtained. Consequently, these techniques have limited ability to produce textured tubular components. In other words, tubes and tube-shaped devices or components that can be or are currently made by mechanically consolidating precursor powders to form tubes are typically manufactured without inducing significant orientation to the constituent particles or grains.
Because randomly oriented components do not take advantage of anisotropic material properties, improvements to these tubular components may be made by inducing a preferred orientation of the powder particles within the tube. Many material properties are dependent upon the crystallographic direction orientation of the grains which constitute the bulk material. Aligning these grains in a desired direction takes advantage of the direction dependent properties. Some common direction dependent properties include fracture toughness, yield strength, current carrying capability, and piezoelectric effects. Production of textured tubes has potential advantages in fracture toughness, critical current, elasticity, and other crystal direction related properties of materials. Improving these properties through texturing allows improved efficiency and/or a reduction in the necessary quantity of material required compared to non-textured components.
Another known manner of producing tubes is disclosed in U.S. Pat. No. 2,962,790, which involves centrifugally slip-casting a ceramic material. While teaching a method to provide a tube shaped product, this reference fails to expressly provide for higher levels of texturing and density which are desirable from the standpoint of improving the properties of tubes formed from anisometric materials having anisotropic properties. In another known process disclosed in U.S. Pat. No. 4,937,214, centrifugal forces are utilized to properly orient the grains of ceramic materials. However, because the cast material does not employ a slip carrier fluid which during casting passes through settled particles and into the mold, texture and density of the cast product are not further increased.
One beneficial application of tubular components involves superconductors. Tubular superconducting devices may be used to protect sensitive electronic equipment against stray electromagnetic fields. Tubular superconducting devices are also used to conduct electrical current more efficiently than metal wires, as such superconducting tubes, when cooled to very low temperatures by fluids flowing therein, have minimal to zero resistance. One relatively new type of superconducting material is known as BSCCO, which advantageously functions at temperatures higher than many longer known superconductors. As BSCCO can be cooled to an operational temperature with liquid nitrogen rather than the significantly more expensive and less available colder liquid helium required for many longer known superconductors, BSCCO tubes may be cheaper to operate than other superconducting tubes. However, because BSCCO is composed of anisometric particles having anisotropic properties, texturing of these tubes is important to produce a tube which functions efficiently.
Thus, it is desirable to provide a process for producing tubes which obtains a highly textured and densified tube.
Centrifugal slip casting utilizes component production and powder particle orientation methods inherent to conventional slip casting, but also involves additional g-loading to produce higher green densities and strengths and improves texturing of the powder particles. Consequently, tubes produced by centrifugal slip casting from anisometric particles having anisotropic properties may be advantageously provided with superior overall properties. The present invention allows superconducting BSCCO material to be formed into a useful tubular shape in a highly textured arrangement, and this texturing increases the efficiency and usefulness of the BSCCO tubes.
In one form thereof, the present invention provides a process for centrifugal slip casting a hollow tube and involves the steps of rotating a hollow tube mold about its axis at a speed sufficient to produce a centrifugal force sufficient to force an introduced slip radially outward toward the inner surface of the mold; introducing a slip into the rotating mold such that the centrifugal force results in the slip migrating radially outward toward the mold inner surface, wherein the slip comprises a carrier fluid and a suspended powder, wherein the suspended powder comprises powder particles having large dimensional aspect ratios; continuing the rotation of the mold after slip introduction at a speed sufficient to maintain the slip outward toward the mold inner surface such that the powder particles suspended within the carrier fluid settle in a textured fashion toward the mold surface and such that the carrier fluid passes by capillary action radially outward around the settled particles, through the inner surface, and into the mold, whereby a cast tubular greenbody is formed of settled powder particles; stopping rotation of the mold; and removing the greenbody from the mold.
In another form thereof, the present invention provides a process for forming a hollow tube comprising the steps of rotating a hollow tube mold about its axis at a speed sufficient to produce a centrifugal force sufficient to force an introduced slip radially outward toward the inner surface of the mold; centrifugally slip casting a first slip within the rotating mold such that the first slip casting forms a first settled powder layer against the mold inner surface, wherein the first slip comprises a suspended powder including a mold release material; centrifugally slip casting a second slip within the rotating mold such that the second slip casting forms a second settled powder layer disposed radially inwardly of the first layer; stopping rotation of the mold; and removing the casting comprised of the first layer and the second layer from the mold.
In still another form thereof, the present invention provides a highly textured hollow tube prepared by the process of rotating an absorbent hollow tube mold about its axis at a speed sufficient to produce a centrifugal force sufficient to force an introduced slip radially outward toward the inner surface of the mold; centrifugally casting a slip within the rotating mold, wherein the slip comprises a carrier fluid and a suspended powder, wherein substantially all of the suspended powder comprises powder particles having large dimensional aspect ratios, wherein during centrifugal casting the powder particles settle in a textured arrangement toward the mold inner surface, wherein during centrifugal casting and after powder particle settlement the carrier fluid further textures the settled particles by passing by capillary action radially outward around the settled particles, through the inner surface, and into the mold to be absorbed thereby, whereby a cast tubular greenbody is formed of settled powder particles; stopping rotation of the mold; and removing the greenbody from the mold.
One advantage of the present invention is that in comparison to traditional slip casting and mechanical consolidation methods, higher greenbody densities and higher textures can be obtained due to the high degree of orientation of particles.
Another advantage of the present invention is that the use of a casting release layer allows the casting to be readily removed from the mold prior to the high temperature sintering required for many cast metals and ceramics, thereby preventing possible melting of the mold or contamination of the casting by diffusion of materials to and from the mold.
Another advantage of the present invention is that melting or partial melting of the cast material to achieve a high degree of texturing or bulk density in the product is not required, thereby eliminating possible phase changes possible with melt or partial melt processing.
Another advantage of the present invention is that a BSCCO tube can be made wherein the BSCCO particles are highly oriented, thereby resulting in a BSCCO tube with properties superior to BSCCO tubes formed using more conventional techniques.
Another advantage of the present invention is that the casting of laminates allows incorporation of multiple layers into the cast tube for enhanced mechanical properties of the tube
Still another advantage of the present invention is that very thin layers of material can be incorporated into the laminate casting.
Still another advantage of the present invention is that densification can be engineered to occur mostly in the through thickness direction with highly anisometric particles, which decreases the stress imposed on the casting during sintering and thus improves the mechanical integrity of the casting.
Still another advantage of the present invention is that the sintering rate of the casting can be controlled by varying the constituent powder particle size distributions.